# Charging a battery by moving a magnet randomly in 2D plane?

I'm sorry if this question is a little abstract, but would appreciate any guidance! It's for a DIY project I am considering.

Suppose you have a flat panel mounted on a wall, and a very strong magnet that will move in any direction on the plane of the flat panel. Through any possible arrangement of conductive wiring in the same plane as the panel (embedded in the panel), is it possible to generate sufficient EMF to charge a battery?

My grasp on this is limited, but I think that it is possible, but I'm unsure of the random motion aspect of it. Is there a possible arrangement of the coil that would result in a consistent EMF to charge a battery regardless of the random 2D motion the magnet makes? If so, what would be the most effective arrangement that maximizes power with minimum coil material for any motion?

If the question seems a bit complicated, any reference to the right direction/method I would use to solve this would also be greatly appreciated!

Borek
Mentor
Is there a possible arrangement of the coil that would result in a consistent EMF to charge a battery

I can be wrong but my take is that it is highly unlikely you will be able to obtain a consistent EMF, but if the EMF is high enough it can be used to power a rectifier capable of smoothing out the output.

CWatters
Homework Helper
Gold Member
One possible issue is that the field of a magnet is closed. Eg the field near one pole loops around to the other pole. So the field would pass through a single coil in both directions cancelling out.

I think you would need a grid of wires in the plane dynamically reconfigured so that those near the pole are connected to the collecting circuit while those further away where the field is in the other direction are disconnected. Something like that.

rbelli1
Gold Member
Can you expand on the details of the situation you are trying to harvest energy from?

BoB

anorlunda
Since you're wanting to charge a battery this is really about power transfer. i.e. how much power moves the magnet and how much of it can be harvested at the battery. The answer will depend on a lot of things but it can be possible. For example: shake powered flashlights do exist. https://en.wikipedia.org/wiki/Mechanically_powered_flashlight#Shake_type_design

I would start with a basic dimensional analysis to figure out if your application makes sense. A reference like this has the equations and figures you need to figure it out. http://physics.gsu.edu/hsu/LCh23.pdf

anorlunda
Can you expand on the details of the situation you are trying to harvest energy from?

BoB

I'm trying to use an magnetophoretic film (like the ones from these toys: https://www.amazon.com/dp/B005NJT4S2/?tag=pfamazon01-20
and figure out a way to generate power through the user's drawing movements, but at a much larger scale than the toy. The end of the pen has a magnet which attracts negatively charged pigment particles out of the white suspension medium in the board.

I can be wrong but my take is that it is highly unlikely you will be able to obtain a consistent EMF, but if the EMF is high enough it can be used to power a rectifier capable of smoothing out the output.

Yeah, I thought that the EMF might be wildly inconsistent / in opposing directions depending on the movement. Thanks for your input!

One possible issue is that the field of a magnet is closed. Eg the field near one pole loops around to the other pole. So the field would pass through a single coil in both directions cancelling out.

I think you would need a grid of wires in the plane dynamically reconfigured so that those near the pole are connected to the collecting circuit while those further away where the field is in the other direction are disconnected. Something like that.

I understand where you're coming from - if you had a simple bar magnet, however held it vertically from the plane such that the S end was further from the plane than the N end, wouldn't the field in one direction be stronger? The other issue arises which would be the direction of nearby coils in the grid, which might produce opposing currents in the coil right? Please correct me if I'm wrong, just speculating. Thanks for the reply.

Since you're wanting to charge a battery this is really about power transfer. i.e. how much power moves the magnet and how much of it can be harvested at the battery. The answer will depend on a lot of things but it can be possible. For example: shake powered flashlights do exist. https://en.wikipedia.org/wiki/Mechanically_powered_flashlight#Shake_type_design

I would start with a basic dimensional analysis to figure out if your application makes sense. A reference like this has the equations and figures you need to figure it out. http://physics.gsu.edu/hsu/LCh23.pdf

Thanks for the links eq1 - I've done some courses in electromagnetism and understand the fundamental mathematics behind it. From a very simple 1D model where you have a conductive wire going along the x-axis, if you move a bar magnet around near the wire, a current will be produced in the wire. This is my core assumption, but I'm not sure if its true based on what CWatters said. Extending it to a 2D model, if you had wires running in X and Y directions, moving a charged particle would produce a current in these wires. Can I replace a charged particle with a bar magnet or is the effect for my model not the same?

rbelli1
Gold Member
and figure out a way to generate power through the user's drawing movements,

What is it that you want to use the energy for?

BoB

What is it that you want to use the energy for?

BoB

Because it's a magnetophoretic board, I want to be able to store this energy, and release it to quickly "wipe" the board by running a charge through the board. By drawing energy from the act of drawing on the board itself, I might be able to implement the "erase" and other functions without external power.

CWatters
Homework Helper
Gold Member
Thanks for the links eq1 - I've done some courses in electromagnetism and understand the fundamental mathematics behind it. From a very simple 1D model where you have a conductive wire going along the x-axis, if you move a bar magnet around near the wire, a current will be produced in the wire. This is my core assumption, but I'm not sure if its true based on what CWatters said. Extending it to a 2D model, if you had wires running in X and Y directions, moving a charged particle would produce a current in these wires. Can I replace a charged particle with a bar magnet or is the effect for my model not the same?

Yes that will produce something. What you can't do is have the magnet entirely within the coil as all the flux would be in the coil all the time. That's what I was trying to say earlier.

With a grid of wires you can use Faraday's law to work out the voltage induced in each wire as the magnet passes.

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/genwir2.html

I suspect you will need multiple turns just to generate enough to rectify it with diodes.

I think a small solar panel charging a battery might be easier.

anorlunda
Staff Emeritus
Thanks for the links eq1 - I've done some courses in electromagnetism and understand the fundamental mathematics behind it. From a very simple 1D model where you have a conductive wire going along the x-axis, if you move a bar magnet around near the wire, a current will be produced in the wire. This is my core assumption, but I'm not sure if its true based on what CWatters said. Extending it to a 2D model, if you had wires running in X and Y directions, moving a charged particle would produce a current in these wires. Can I replace a charged particle with a bar magnet or is the effect for my model not the same?

You're being sloppy and thus confusing the question.

There are two parts to your question.
1. Qualitative: Can you induce current in a wire with a moving magnet?
2. Quantitative: Is that enough to charge a battery? That's what you asked in your original post. You didn't specify what kind of battery, or whether you mean charge it completely, or just to begin charging at all.
The answer to 1 is yes.

The answer to 2 must be yes in some circumstances. We could never have electric generators otherwise. So your question reduces to just the quantitative. Do the circumstances you describe transform enough energy to charge a battery? (whatever that means)

I'm afraid that you would have to be more exact in the description to offer a good answer. Making it more complex, such as going from 1D to 2D just obscures the basic question.

Give us the dimensions and strength of the magnet. It's orientation and speed. Specify the gauge and exact layout of the wires and their connections to the load. Specify the battery, and the state of charge required. If you do that, and if you understand Faraday's Law and Lenz's Law as you said, then you should be able to calculate the answer yourself.

Without that, the answer to the qualitative question is "It depends."

A non-mathematical approach to the qualitative question was suggested by @eq1 in post #5. Those shake-to-light flashlights are analogous to your idea. That is a voltage and power level in the same order of magnitude as charging a battery. Look at the magnets and wires, and the battery, inside those flashlights, consider the vigor needed to shake them, and consider how quickly the LED goes out when you stop shaking (about 1 minute shaking gives 10 seconds light), then imagine how many hours it might take to completely charge the battery, and you have a guess at your answer.

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jim hardy
If you conceal an array of flat coils behind the panel, each with its own rectifier, then waving a powerful magnet in front will generate a small amount of power. Analogy is a wind-up torch, where you crank a tiny motor as a generator. Efficiency would be very, very low, of course.

FWIW, you might find you're harvesting micro-power without the magnet due to RFI from house wiring, local radio station, WiFi etc etc. Think wireless charger...

This used to be a regular 'science fair' project, an LC tuned to powerful local radio station rectified to micro-power a one-transistor amplifier for a distant station...

rbelli1
Gold Member
That is a voltage and power level in the same order of magnitude as charging a battery. Look at the magnets and wires, and the battery, inside those flashlights, consider the vigor needed to shake them, and consider how quickly the LED goes out when you stop shaking (about 1 minute shaking gives 10 seconds light), then imagine how many hours it might take to completely charge the battery, and you have a guess at your answer.

I have one of those flashlights. One minute of shaking charges the capacitor, not battery, enough to give a couple seconds of useful light then many minutes of glow that is not particularly useful.

The easiest way to determine if the OP's idea would work would be to build one for erase only. Measure how much energy is needed to erase one time. Next estimate how much energy can be stored with one drawing. If the numbers are close then you have some confidence to build a real charging one to test out.

BoB

Tom.G